1. Field of the Invention
The present invention relates to request transmission methods and more particularly, to a method of determining request transmission priority subject to request source and transmitting request subject to such request transmission priority in application of Fieldbus communication framework, whether the external requests came from the source that has the priority right, and then transmits every request subject to the priority order, preventing the slave device from receiving an external request from the master device at a late time.
2. Description of the Related Art
Following fast development of communication technology, communication networks have also been well developed. LAN (Local Area Network) and WAN (Wide Area Network) are the original categories of networks categorized subject to their scope and scale. A LAN connects network devices over a relatively short distance. A WAN is a geographically-dispersed collection of LANs. The Internet is the largest WAN, spanning the Earth.
Either in LAN or WAN, Ethernet has proven itself as a relatively inexpensive, reasonably fast, and very popular LAN technology. Ethernet cables (fiber optics or twisted pair) are limited in their reach, and these distances are insufficient to cover medium-sized and large network installations. Therefore, repeaters are used in Ethernets. A repeater in Ethernet networking is a device that allows multiple cables to be joined and greater distances to be spanned. A bridge device can join an Ethernet to another network of a different type, such as a wireless network. Popular types of repeater devices are Ethernet hubs, switches and routers. By means of the combination of Ethernet cables with hubs, switches and/or routers, an Ethernet networking allows transmission or control of data or instructions among different LANs, computers, and/or other devices such as surveillance systems, security systems, automation systems, etc.
Further, there is known a network communication architecture commonly seen in industrial control and device communication protocol or rules, i.e., Fieldbus”.
Fieldbus is usually used on the industrial network system for real-time distributed control. Its basic configuration is to handle management through a central control unit so that an operator can monitor or program the whole system. The central control unit is typically linked to a middle layer of programmable logic controllers (PLCs) via a bus system. At the bottom of the control chain is the fieldbus which links the PLCs to the components which actually do the work such as sensors, electric motors, switches and contactors.
Following alternation of generations, there are now a wide variety of concurring fieldbus standards. Some of the most widely used ones include CAN, EtherCAT, DeviceNet, Modbus, PROFIBUS, SECS and CompuBus.
Further, Fieldbus is a new industrial digital communications network intended to replace the existing 4-20 mA analogue signal standard. The network is a digital, bi-directional, multidrop, serial-bus, communications network used to link isolated field devices, such as controllers, transducers, actuators and sensors. Each field device has low cost computing power installed in it, making each device a “smart” device. Each device will be able to execute simple functions on its own such as diagnostic, control, and maintenance functions as well as providing bi-directional communication capabilities. With these devices not only will the engineer be able to access the field devices, but they are also able to communicate with other field devices for master/slave communication. The major advantage of the Fieldbus that is most attractive to the end user is its reduction in capital costs. The savings attained by the user stem from three main areas, initial savings, maintenance savings, and savings due to improved systems performance. One of the main features of the Fieldbus is its significant reduction in wiring. Only one wire is sufficient to build a Fieldbus system with varying number of devices. The cost of installing field equipment in a Fieldbus system is thus significantly reduced. Installation costs are further reduced due to the fact that the Fieldbus it is a multi-drop rather than point-to-point system and the multidrop network can offer a great reduction in field wiring expense. The fact that the Fieldbus system is less complex than conventional bus systems implies that there will be less overall need for maintenance.
FIG. 3 is a schematic drawing illustrating multiple requests receiving status of a communication gate according to the prior art. Under the Fieldbus architecture, when the master device or communication gateway A received multiple requests, it transmits the requests to the slave device B subject to the order of FIFO (first in first out), i.e., the master device or communication gateway A handles the received requests subject to the order of their received times, but is unable to arrange or transmit the received requests subject to order of their importance level.
Because the master device or communication gateway A handles received requests subject to the order of FIFO (first in first out), it may be unable to transmit a request to the slave device B in time. When communication traffic is low, a communication gateway can always handle all received requests in time. However, when traffic gets higher, the gateway needs much more time to handle received requests in received time order. If there is an important request arrived at this time, the gateway may unable to transmit it to the slave in time since there are too many un-handled requests received before. This could result in many problems. For example, in a regular industrial manufacturing line (such as wafer plant, plastic materials plant), when a request to alter the set values of a slave device B (for example, the revolving speed of a motor) is given to the communication gateway A, an abnormal or interruption of the manufacturing line may occur if the slave device B cannot receive the accurate request in time to execute the request on accurate time, thereby resulting in paralysis or damage of the machinery of the industrial manufacturing line. Further, in a building control, the temperature control, emergency exit system and other control actions need to make an accurate response at the accurate time point. If responding tactics cannot be started at the accurate time point, it will get no help in reducing the probability of occurrence of disaster and the loss resulted from the disaster.
Therefore, it is desirable to provide a method of handling requests according to priority of importance.